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This arrangement features two converters situated at a single site, eliminating the need for a d.c transmission line The valves for both converters can be housed in a shared valve hall or integrated into a single structure, with options for outdoor placement as well Additionally, components such as the control system, cooling equipment, and auxiliary systems may be consolidated in one area or designed in a layout that serves both converters Various circuit configurations exist, as illustrated in Figure 2, and their performance and economic implications require careful evaluation Notably, d.c filters are unnecessary in this setup.
Figure 2 — Examples of back-to-back HVDC systems
To optimize system costs, including loss evaluations, voltage and current ratings should be carefully selected for a given power rating Users typically do not need to specify these ratings unless necessary for compatibility with existing systems or future expansions While a 12-pulse converter unit is generally preferred for economic reasons, it is not a strict requirement In scenarios where the failure of one converter unit does not compromise overall power capability, large HVDC substations may consist of multiple back-to-back systems For cost efficiency, some equipment from these systems can be co-located or integrated, although care must be taken to avoid failures that could impact all units.
+AMD2:2017 CSV © IEC 2017 back-to-back systems need to be carefully considered and preventive measures taken where appropriate.
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Cost considerations often lead to the adoption of a monopolar HVDC system with earth return system (Figure 3), particularly for cable transmission which may be expensive iI IK
Figure 3 — Monopolar HVDC system with earth return-system
The monopolar earth return configuration can serve as the initial phase in developing a bipolar scheme This setup may consist of one or more 12-pulse units arranged in series or parallel at the ends of the HVDC transmission Utilizing multiple 12-pulse units offers several advantages, including maintaining partial transmission capacity during converter unit outages, allowing for staged project completion, and addressing the physical constraints associated with transformer transport.
The configuration necessitates the installation of one or more d.c reactors at both ends of the HVDC overhead line or cable, typically positioned on the high-voltage side Alternatively, these d.c reactors can be split into two sections, with one part on the high-voltage side and the other on the earth side, provided that the performance remains satisfactory, particularly in large-scale ultra high voltage direct current (UHVDC) converter systems.
When dealing with overhead lines, it is essential to install d.c filters at both ends, as outlined in Clause 17 Additionally, a reliable earth electrode line and a continuously operable earth electrode must be established at each end of the transmission, taking into account factors such as corrosion and magnetic field effects.
Figure 4 — Two 12-pulse units in series
Figure 5 — Two 12-pulse units in parallel
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The configuration depicted in Figure 6 serves several key purposes: it acts as the initial stage in developing a bipolar system when long-term earth current flow is undesirable, is suitable for short transmission lines where constructing earth electrode lines is economically unfeasible, addresses situations with high earth resistivity that could lead to significant economic penalties, and meets environmental and safety standards by preventing long-term earth current flow.
This configuration employs a high-voltage conductor alongside a low-voltage conductor The neutral connection at one HVDC substation links to its station earth or the corresponding earth electrode, while the neutral at the other HVDC substation connects to its station earth via a capacitor, an arrester, or a combination of both.
DC reactors are essential at both ends of high-voltage conductors, although they can be positioned on the earth side if performance standards are met Additionally, DC filters may be required for overhead HVDC transmission lines.
If this configuration is the first stage of a bipolar system, its neutral conductor could be insulated for the high voltage at this stage of development
In a metallic return scheme, DC fault current can enter the AC system and return through the neutral point of transformers at the converter station, potentially causing protective relays in nearby stations to malfunction due to core saturation from the DC current To mitigate these issues, the effective solution is to insert a neutral grounding resistor with low resistance into the transformers at the converter station.
Figure 6 — Monopolar HVDC system with metallic return-system
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The most common configuration for a d.c transmission line connecting two HVDC substations includes electrodes for earth return operation, resembling a double-circuit a.c transmission This arrangement minimizes harmonic interference compared to monopolar operation and maintains low earth current flow Additionally, combining two monopolar earth return schemes can effectively create a bipolar scheme.
In a unidirectional power flow, one pole maintains a positive polarity to the earth while the other exhibits a negative polarity Conversely, when the power flow reverses, the polarities of the two poles switch Operating both poles simultaneously allows for the unbalanced current in the earth path to be minimized effectively.
IEC TR 60919-1:2010+AMD1:2013 +AMD2:2017 CSV © IEC 2017
Figure 7 (b) — Rigid bipolar HVDC system y |
This configuration includes several emergency operating modes that must be addressed in the specifications Firstly, during the outage of one HVDC transmission line pole, the converter equipment of the remaining pole must maintain continuous operation using earth return Additionally, if long-term earth current flow is not preferred and the defective line pole has some low-voltage insulating capability, the bipolar system should operate in monopolar metallic return mode To transition to this emergency mode, the conductor of the inactive pole is connected in parallel with the earth path, followed by interrupting the earth path to redirect current to the metallic path via the inactive pole's conductor To ensure uninterrupted load transfer, a metallic return transfer breaker (MRTB) is required at one terminal of the DC transmission.
In scenarios where brief power interruptions are acceptable, the use of a Multi-Return Transformer Block (MRTB) may be unnecessary The neutral equipment at the MRTB end of the High Voltage Direct Current (HVDC) transmission system must be insulated from the earth at a higher voltage than the opposite end During maintenance of the earth electrodes, the bipolar system should remain operational with the station neutrals connected to the station earth, provided that the unbalance current between the two poles is minimized to prevent saturation in the converter transformers If one transmission line is lost, both poles should automatically block In bipolar operation with both earth electrodes connected, the HVDC system must accommodate significantly different currents in each pole, especially under conditions like cooling loss For cases of partial line insulation damage, converters should be designed for continuous operation at reduced voltage, allowing either pole to function at lower voltage levels Additionally, if one transmission line pole fails, the two substation poles can be connected in parallel using switches for polarity reversal, enabling monopolar earth return mode operation, which necessitates that the d.c terminals are insulated for full pole voltage and that the line and earth electrode can handle higher currents than normal.
Each pole of a high-voltage direct current (HVDC) system requires one or more d.c reactors, typically positioned on the high-voltage side For large-scale ultra high voltage direct current (UHVDC) converter setups, these reactors can be split between the high-voltage and earth sides if performance remains satisfactory Additionally, if the HVDC system incorporates an overhead line, d.c filters are generally necessary The most common configuration is a 12-pulse unit per pole, although larger capacity systems or those undergoing staged expansion may necessitate the use of multiple 12-pulse units arranged in series or parallel.
The majority of HVDC systems use an electrode line or metallic return conductor for the direct current return path However, if balanced bipolar operation is consistently maintained, these components can be omitted This configuration is known as the "rigid bipole HVDC system," which, while limiting operational modes, can significantly lower installation costs.
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4 MRTB Metallic return transfer breaker
Figure 8 — Metallic return operation of the unfaulted pole in a bipolar system
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In situations where earth currents are intolerable, the distance between HVDC system terminals is short, or high earth resistivity prevents the use of an earth electrode, a third conductor can be added to create a bipolar HVDC system with a metallic return This third conductor carries unbalanced currents during bipolar operation and provides a return path when one pole of the transmission line is out of service It requires only reduced voltage insulation and can also function as a shield wire if the line is overhead If fully insulated, it can serve as a spare conductor, necessitating a separate shield wire.
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Figure 9 — Bipolar HVDC system with metallic return-HVDC-system
In HVDC transmission systems, one substation's neutral must be grounded, while the other substation's neutral can either float or connect to its station earth using an arrester, a capacitor, or a combination of both.
The system can operate in bipolar mode even if one conductor is unavailable, provided the third conductor is fully insulated It is essential to connect the neutrals at both terminals to their local station earths and maintain low unbalanced current flow If one pole is lost, the other pole must be blocked until the necessary switching is completed to ensure the remaining functional parts of the HVDC transmission system can operate effectively.
If one substation pole becomes unavailable, the system can be operated in monopolar metallic return mode by utilizing the other substation pole This configuration is also called a
In a metallic return scheme, d.c fault currents can enter the a.c system and return through the neutral point of transformers at the converter station, potentially causing protective relays in nearby stations to malfunction due to saturation from the d.c current To mitigate this issue, the installation of a neutral grounding resistor with low resistance on the transformers in the converter station is an effective solution.
+AMD2:2017 CSV © IEC 2017 3.7 Two 12-pulse groups per pole
For high power ultra high-voltage direct current (UHVDC) converter systems, utilizing two 12-pulse units per pole is often more effective in meeting the required ratings This approach prevents excessive dimensions and weight of the converter equipment, particularly the converter transformer, which would be significantly larger if only a single 12-pulse unit per pole were implemented.
Two 12-pulse converters can be connected in series (Figure 10) or in parallel (Figure 11), and the selection of converter arrangement depends on the specific requirements of the project
On the other hand, if a project requires reduced voltage operation, for instance, due to occasional salt contamination, then series option-sheuld be-selected may be preferred
In the event of a forced or scheduled outage of a 12-pulse converter, both series and parallel configurations experience a 25% loss of capacity, assuming identical power-rated converters are used If there is adequate overload capability, nearly full power can be restored In a series configuration, the two poles can continue to operate with balanced current, avoiding earth current issues, although a bypass switch is necessary for each 12-pulse converter Conversely, in a parallel configuration, the two poles may operate with unbalanced current during an outage, resulting in significant current flowing through the earth.
The expense of utilizing two 12-pulse groups per pole arrangement is anticipated to be higher than that of a single 12-pulse group per pole for an equivalent total rating, leading to increased complexity in the control system.
For large bipole capacities, utilizing two 12-pulse groups in series per pole allows for minimal capacity loss during a forced or scheduled outage, as only 25% of the capacity is affected This configuration enables the two poles to maintain balanced current without earth current or operate with unbalanced current when connected in parallel If sufficient overload capability is present, nearly full power can be restored Additionally, this setup offers advantages such as a soft start and stop sequence, along with flexible utilization of the HVDC system through various combinations of converter groups.
DC switches are essential for bypassing and deactivating any 12-pulse group However, the cost of implementing this setup is anticipated to be greater than that of using one 12-pulse group per pole for the same total rating.
A 12-pulse converter necessitates two three-phase transformer valve windings, comprising one star-connected and one delta-connected This configuration can be achieved through various setups: a single three-phase transformer with two valve windings, two three-phase transformers with one in star-star and the other in star-delta, three single-phase transformers each featuring two valve windings for star and delta connections, or six single-phase transformers arranged in two three-phase banks, with one bank in star-star and the other in star-delta.
Depending on the availability requirements of the HVDC system, spare transformers may be necessary at one or both ends If a three-phase transformer with two valve windings is utilized, only one spare unit is needed Due to the differing designs of star- and delta-connected three-phase transformers, it is advisable to have one spare for each design For single-phase, double-valve winding transformers, only one spare is required since all three are identical Lastly, the option of using single-phase transformers with star- and delta-valve windings would necessitate two spare transformers, one for each design.
In the absence of spare transformers, options b) and d) enable six-pulse operation at half-power during a transformer outage, provided the HVDC system is designed for this operational mode and the harmonic conditions for both a.c and d.c are acceptable However, six-pulse operation cannot be achieved with options a) and c).
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Figure 10 — Bipolar system with two 12-pulse units in series per pole
Figure 10, with d.c switch 3 (named as: MRTB and GRTS), is usually valid for rectifier station The d.c switch 3 is not necessary for the inverter station.
Figure 11 — Bipolar system with two 12-pulse units in parallel per pole
In parallel connections, it is not always necessary to separate d.c reactors, as their quantity and configuration are determined by system studies and design outcomes Additionally, converter transformers featuring a tertiary winding for reactive power and a.c harmonic filter equipment can be utilized.
There are a number of possible d.c switching arrangements intended to increase HVDC system availability
Monopolar metallic return operation of a bipolar system is discussed in 3.5
In bipolar systems, d.c switching enables the use of any conductor for connecting to substation poles or neutral, which is particularly beneficial for cable schemes with available insulated spare cables or parallel connections This setup allows for the paralleling of cables to minimize line losses when one substation pole is out of service Typically, d.c buses are fixed relative to converters, consisting of two pole buses and a neutral bus, which limits the ability to connect the two substation poles in parallel.
4 DC switches 8 DC line/cable
Figure 12 — DC switching of line conductors
To enable the flexibility of connecting two substation poles in parallel, it is essential to incorporate a provision for polarity reversal in at least one of the poles Additionally, the neutral end of this pole must be insulated to withstand the full line voltage A potential switching arrangement illustrating this setup is depicted in Figure 13.
4 DC switches 8 DC line/cable
Figure 13 — DC switching of converter poles
If a HVDC transmission system includes both overhead line and cable sections, a d.c switching arrangement such as in Figure 14 may be used at the junction of the overhead and cable sections.
4 DC cables (two poles, one spare)
Figure 14 — DC switching — Overhead line to cable
For more than one bipolar line, paralleling of converter poles may be considered, in order to allow restoration of transmission capability (Figure 15) for transmission line outages
For long bipolar lines in parallel, intermediate switching such as in Figure 16 may be provided
3.10 Series capacitor compensated HVDC systems